The present invention relates to a selective ink jet printing apparatus comprising a duct terminated at one end by a nozzle, the duct comprising a first portion of substantially rigid material carrying the nozzle, and arranged to be selectively conditioned by a transducer to generate a pressure wave which causes a droplet of ink to be expelled through the nozzle, and a second portion of viscoelastic material, arranged substantially to absorb the energy of said pressure wave propagating back within the duct.
As is known, a pressure wave in a liquid in a duct gives rise to reflection phenomena both at the end of the duct and in discontinuities in the duct itself. In addition, by virtue of the physical and geometrical characteristics of the duct and the physical characteristics of the liquid, each duct portion behaves like a system which resonates at a predetermined frequency.
In selective ink jet apparatus, since the period of time between one expulsion operation and the next varies within very wide limits, it is not possible entirely to avoid actuation of the jet at the resonant frequencies of the various duct portions.
U.S. Pat. No. 3,832,579 proposes two ink jet apparatuses in which at least a part of the energy of the pressure wave propagating towards the ink reservoir is absorbed before reaching the reservoir. In one of those two apparatuses, the energy absorption effect is achieved by means of an acoustic resistance which is formed for example by a bunch of glass fibres disposed in an intermediate portion of the duct. In the other apparatus, the intermediate portion of the duct comprises a tube of viscoelastic material, the diameter of which is such as to eliminate reflection phenomena at its connection to the duct portion carrying the nozzle, and the length of which is such as to absorb that part of the energy from the pressure wave.
Another selective ink jet apparatus has also been proposed in European patent application No. 21755, wherein the energy of the wave towards the reservoir is absorbed by a grating or grid which is disposed in the duct in the vicinity of the nozzle to damp oscillation of the meniscus, while a second grating or grid disposed at the opposite end of the duct eliminates reflection of the residual wave.
In the above-mentioned known apparatuses however, no account is taken of either the inherent resonance of the meniscus in the nozzle or the inherent resonance of the duct portion of viscoelastic material, the frequency of which decreases with the length of the duct. Also ignored is the variation in the viscoelastic characteristics of the material of the duct as the ambient temperature varies.
There exists a need for a selective ink jet printing apparatus wherein the energy of a wave of any frequency is substantially absorbed and the resonance phenomena of the individual duct portions are damped.